The present embodiment relates to roller levelers. It finds particular application in conjunction with roller levelers with common back-up rolls, and will be described with particular reference thereto. However, it is to be appreciated that the present embodiment is also amenable to other like applications.
A roller leveler typically includes multiple pairs of offset work rollers or rolls. Different size levelers can have different quantities of work rolls and back-up rolls. The upper rolls are typically offset one-half the distance between a pair of adjacent lower rolls. The metal strip passes between the upper and lower rolls. The number and spacing of the rolls depend on the thickness and strength of the metal strip. Typically, as the strip thickness decreases, the spacing of the rolls, as well as the roll diameter, decrease. As the strip passes between the rolls, it is bent up and down multiple times before it exits the leveler. This reversed bending beyond the yield point of the material is the mechanism whereby the strip is flattened.
Metal is formed into strip by a process known as rolling, wherein the strip is passed between a pair of work rolls of a rolling mill to reduce its cross-sectional thickness. In the process, the strip is elongated and rolling continues until the strip is reduced to the cross-sectional thickness desired. This rolling process may start with heated billets or slabs of metal, wherein the metal is rolled at a very high temperature, or it may start with previously rolled strip wherein the strip is passed between work rolls in the cold state. In either event, when the strip exits from the mill, it may be convolutedly wrapped to form a coil. When the coil has been formed, curvature of the coil tends to stay with the strip when it is necessary to uncoil the strip for further processing. Thus, the primary problem with strip coming off of a coil is the curvature which remains with the strip and which varies throughout the entire length of the coil as a function of the radius of any particular portion of the strip while in the coil. Accordingly, the outer wrap of the coil will have less curvature than an inner wrap. To remove this variable curvature in the strip is one of the purposes of a roller leveler. It is necessary to remove this curvature so that the strip may be cut accurately and rendered suitable for other manufacturing operations, such as punching, drawing, forming and the like. It is well established that the flatter the strip is prior to a subsequent manufacturing operation, the more accurate and satisfactory will be the end product of that operation. Thus, even where portions of steel strip are deep drawn, they do not draw as satisfactorily if the strip initially is not substantially flat before the draw.
In addition to strip curvature, other unwanted properties are sometimes impressed upon the strip during hot and/or cold rolling which render the problem of flattening strip much more complex. In order to reduce cross-sectional thickness of the strip during rolling, it is necessary to force the strip between rolls under tremendous pressure whereby the strip essentially becomes a wedge which tends to separate the rolls. The force of roll separation is dependent upon the physical properties of the strip including width, thickness, hardness, temperature, yield strength, and amount of reduction being attempted during the pass of the strip between the rolls. If the work rolls are not sufficiently supported by back-up rolls, it is possible for the strip to actually cause the work rolls to bend at their centers, wherein the resultant strip cross-sectional shape is thicker in the middle than at the edges. Strip rolled with thicker center portions indicates that greater pressure has been applied to the edges of the strip than at the center, thereby causing the edges to elongate at a greater rate than the center of the strip. Because this excess metal on the edges must go somewhere, but is restrained by the center, the result usually is a product having what is referred to as edge waves. In other words, the center of the strip is relatively flat longitudinally, but the edges of the strip are sinusoidal.
Just the opposite may occur during rolling of strip, wherein the rolls may be so reinforced, or may be so contoured, that they resist or otherwise offset the wedge effect of the strip. However, if the rolls are over compensated against roll bending, the resultant is strip that is rolled thinner in the center than at the edges. In this circumstance, the center of the strip tends to become elongated, producing a condition sometimes referred to as “oil canning”. By this is meant that the elongated center portion of the strip compensates for this elongation by bulging either up or down. The result is strip that can literally be snapped up and down like the bottom of an oil can because of the stresses set up by this localized elongation.
A metal strip product is fed into a roller leveler, typically from a coil. Roller levelers use multiple work rolls to flatten the strip as it passes through the leveler. The path of the strip passes between offset upper and lower work rolls, in effect reverse bending the strip multiple times before the strip exits the leveler.
A typical roller leveler is designed to process a range of strip thicknesses and strip yield strengths. As the strip passes between the work rolls, very high separating forces are generated against the work roll face, yet the work roll diameters are of necessity relatively small; this is to allow the work rolls to bend and to space them close enough to properly work the strip. The work rolls are supported by flights or groups of back-up rolls. The back-up rolls support the work rolls and prevent them from incurring excessive bending in reaction to the separating forces.
If a process line requires a wider range of strip thicknesses, two levelers can be used, or alternatively, one leveler can be supplied with multiple work roll diameters and spacings. These machines are typically called cartridge or cassette type levelers. In the cassette style machine, present practice requires that a cassette consist of the upper and lower work rolls, as well as the corresponding upper and lower back-up rolls. Thus, this combination of stacked rolls, including the back-up rolls, is removed and injected into the leveler as a unit, which increases costs, time and maintenance of the leveler unit.
Thus, there is a need for a roller leveler with common back-up rolls which does not require the removal of the back-up rolls from the leveler and overcomes the above-mentioned deficiencies while providing better and more advantageous overall results.